A gear is generally fastened to a counter-joint component by press-fitting wherein a small-diameter boss formed at an axial end of the gear is press-fitted into a circular opening for press-fit which is formed in the counter-joint component. Such a method is disclosed in JP 8-247223 A, for example. When great fastening strength is required, for example as in a case where a gear is concentrically fastened to a rotational output shaft, a key, a knock pin, and so forth are used for securing enough fastening strength of the gear.
However, in some cases, design-wise it is impossible to have a space for installing such a key, a knock pin, and so on.